In offshore regions applying vessels for deploying and retrieving heavy equipments is well known. Such heavy equipments are known to relate to oil and gas exploration and production, mining and mineral exploration, drilling operations and so on.
For example, WO 2010/020026A2 discloses a double hall catamaran type vessel having a deck and a common bridge for loading and unloading operations of supplies, in the oil sector. However, this vessel is not suitable for offshore areas where waves go up to even ten meters high, such as in the North Sea. In these areas, where the sea is known to be hostile, mono hull off shore vessels have been tried such as the ones disclosed in WO 2009/102197 and WO 2009/102196. However, over the years it has been observed that these mono hull off shore vessels, such as disclosed in these two publications, are not suitable for many off shore operations, particularly in hostile seas, such as in North Sea, Brazil and Gulf of Mexico having regard to stability and safety considerations.
Semi-submersible vessels for use in various types of offshore work are known in the art. These vessels are particularly useful in offshore regions where the sea is hostile. It is also known, that these semi-submersible vessels are designed to take care of safety and stability considerations, which are of prime importance in such hostile offshore areas. For example, WO 99/12807 discloses a semi-submersible vessel design which provides a strong and substantially rigid base to support the deck(s) of the superstructure. WO 2009/084950, discloses semi-submersible vessels which are braceless. WO85/03050 discloses a geometrically improved semi-submersible vessel having a buoyant centre column centrally disposed about the drilling central string. This design was meant to significantly reduce heave motion under sea states. WO 99/57011 discloses a design of a semi-submersible vessel which ensures safety of the mineral exploration platform it supports. WO 2007/097611 discloses a semi-submersible vessel which has an assembly of hull section, support structure and deck structure, the deck structure having reinforcement for surviving storms.
However, certain vital disadvantages have been observed in respect of the vessels in the preceding paragraph and similar such vessels. Primarily, designing such vessels involve very high costs and long project implementation time for observing proper safety considerations in hostile weather and sea conditions. Further, those involve deployment of large cranes and lifting gears which add on to the costs and operational inconveniences. Additionally, existing vessels have variable drafts in harbours and are basically barges with too much motion and become unreliable in extremely hostile weather, despite all manoeuvrings. In addition, these vessels are not ship shaped and so cannot perform effectively as normal sailing vessels in non-submerged conditions, because it has almost been an accepted fact that ship-shaped vessels are less suitable for many offshore operations. Furthermore, deployment and retrieving of heavy equipments applying these vessels result in that the equipments and lifting/lowering devices are subjected to the dynamical forces in the splash zone.
Such vessels as referred to in the preceding paragraph, are also not known to have an optional temporary port which function on an identical principle as the main vessel, for accommodating crew vessels/supply vessels/any other vessels for safe transfer of crew and material. U.S. Pat. No. 5,215,024 does disclose an open ocean based berthing facility for capturing a ship or similar vessel, but it is mainly directed to serve the purpose of sea bases for defence purpose. The berthing facility has a buoyant platform having an enclosure formed therein for receiving the vessel. Movement of the vessel is coupled to the movement of the platform so that relative motion between the vessel and platform is relatively reduced. This technology does not disclose a semi-submersible vessel for offshore activities, which nullifies/substantially reduces the disadvantages of prior art as disclosed in the previous paragraph and simultaneously, has an optional temporary port which function on an identical principle as the main vessel, for accommodating crew vessels/supply vessels/any other vessels for safe transfer of crew and material. The same observations hold good in respect of the technology disclosed in US2006/0086304 and in WO93/04914. The former discloses a vessel for rescuing vessels in distress. The rescue vessel has an elongated basin and a ballast device. There are two lateral hulls surrounding the basin and delimiting an upper edge of the rescue vessel. When the vessel to be rescued is evacuated, the upper edge of the rescue vessel is above sea level. When a vessel is to be rescued, the upper edge of the rescue vessel is below the keel of the vessel in distress. Although it is stated in the document US2006/0086304, that such rescue vessels may be applied in respect of drilling or production platforms and parts of such platforms, but no clear teaching exists regarding a semi-submersible for offshore activities, which nullifies/substantially reduces the disadvantages of prior art as disclosed in the previous paragraph and simultaneously, has an optional temporary port which function on an identical principle as the main vessel, for accommodating crew vessel(s)/supply vessel(s)/any other vessel(s) for safe transfer of crew and material. Identical observations hold good in respect of WO93/04914 which discloses a jumbo barge carrier fast sealift and port system having a trapezoidal double hull design. It includes a barge-carrying vessel, at least one cargo carrying barge, a transportable port system and a causeway. It relates to rapid transport and deployment of extremely large amounts of cargo needed to meet humanitarian, economic and military contingencies and strictly speaking does not relate to a semi-submersible vessel for retrieving and deploying equipment(s) in offshore region, which is the subject matter of the present invention.
Hence, there was a long felt need to design a semi-submersible vessel which nullifies/substantially reduces the aforesaid drawbacks in general and which in particular is a mono hull column stabilised unit which is cost effective, stable and reliable in extremely hostile sea conditions, is adapted to sail as a vessel with low draft at a fairly high speed in non-submerged condition. There was also a long felt need to develop a semi-submersible vessel, which is ship shaped and is simultaneously adapted to be applied effectively in a wide range of offshore applications. Furthermore, there was long felt need to design a semi-submersible vessel which nullifies/substantially reduces the drawbacks in such known semi-submersible vessels and simultaneously has an optional temporary port which functions on an identical principle as the main vessel, for accommodating crew vessels/supply vessels/any other vessels for safe transfer of crew and material. There was also a long felt need for designing a method for deploying and retrieving heavy equipments from water, in offshore operations without subjecting the equipments and the lifting/lowering devices to the dynamical forces of the splash zone/water line, by submerging and retrieving the section through water, instead of the usual method of lowering it through the water line.
The present invention, meets the above long felt needs and other needs associated therewith and the construction of the mono hull column stabilized semi-submersible vessel as disclosed hereinafter, is consequential to the present invention.